Device manufactured by room-temperature bonding, device manufacturing method, and room-temperature bonding apparatus

ABSTRACT

An inter-substrate material layer is formed between a first substrate and a second substrate to generate a bonding strength. A plurality of metal elements are present in the inter-substrate material layer. An interface element existence ratio of the plurality of metal elements is 0.07 or above. A device can be obtained in which substrates difficult to bond (for example, SiO 2  substrates) are bonded at room-temperature to have practical bonding strength.

TECHNICAL FIELD

The present invention relates to room-temperature bonding, and inparticular, relates to a device manufactured by room-temperaturebonding, a manufacturing method of the device, and a room-temperaturebonding apparatus therefor.

BACKGROUND ART

A MEMS (Micro Electro-Mechanical Systems) device is known, in whichminute electrical parts and mechanical parts are integrated. The MEMSdevices are exemplified by a micromachine, a pressure sensor, and amicro motor. A wafer-level semiconductor device manufacturing process isgenerally employed for manufacturing of the MEMS device. A plurality ofdevices are formed and sealed on a semiconductor wafer at one time, andthe wafer is then divided into individual devices through dicing. In awafer-level packaging step in this manufacturing process, a scalingwafer is located on and bonded to a device wafer to seal and manufacturethe devices.

In recent years, a manufacturing method is proposed which uses a directbonding method for bonding substrates. In the direct bonding method,which does not use adhesive material and solder, another material layeris not present at a bonding interface. For this reason, there is anadvantage that high bonding strength and good interface properties canbe obtained. As an example of a conventional direct bonding method forthe manufacturing of the MEMS device, anodic bonding and diffusionbonding are exemplified. Additionally, a bonding method in which ahydroxyl group is provided to a flattened and cleaned surface to obtainstrong bonding through hydrogen bonding and heat treatment, is alsoproposed in recent years.

With the direct bonding method, however, heat treatment involved in abonding process or a process after the bonding process causes a problemin the manufacturing of the MEMS device. Many MEMS devices are formed bybonding substrates which are determined based on usage situation andmanufacturing requirement. For this reason, there is a case that thermalstrain is caused at a bonding interface due to heat treatment and thatreliability and durability of the device are lost, when coefficients ofthermal expansion of substrates are different. Reduction in temperatureof the bonding process is a great technical theme in the manufacturingof the MEMS device. Another theme on the manufacturing is that a tacttime at the time of manufacturing cannot be reduced since a time istaken for heating and cooling.

From the above views, realization of a room-temperature bonding processis desired which does not involve a heating step in the bonding process.In this case, in the room-temperature bonding process, especially, inwhich the bonding is performed by use of dangling bonds on bondingsurfaces without providing an active group on the surfaces, bondingperformance is greatly affected based on material properties of thesubstrates. An oxide material frequently used as an electronic devicematerial, especially an SiO₂ material (synthetic quartz, glass and soforth), is known as a material difficult in bonding in theroom-temperature bonding process.

Although the room-temperature bonding process was conventionally knownas one of metal bonding processes, an application field of theroom-temperature bonding process has been gradually developed to bondingof semiconductor materials and oxide materials in recent years. In caseof the oxide material such as Al₂O₃, it is known that a certain degreeof bonding strength can be obtained through surface activation andpressure welding, as disclosed in a report by Takagi et al. (NEDOPreliminary Drafts of Research Promotion Project Accomplishment DebriefSession for the 15th year of the Heisei era, pp. 220-225 (2003)).However, in most oxide materials, practical bonding strength has notbeen obtained. For this reason, methods have been proposed which usesurface treatment such as provision of an active group to bondingsurfaces and heat treatment.

Japanese Patent Application Publication (JP-P2004-054170A) discloses abonding method for laser optical crystals. Only ion beam etching isperformed to a bonding surface without using an interface bonding layerof adhesive material, and then the laser optical crystals are bonded.This method has been developed as the bonding method for laser opticalcrystals, especially for YVO₄ crystals. This method can be applied tosome of oxide materials but cannot be applied to SiO₂ material, asmentioned above. In addition, heat treatment after the bonding processcauses a problem in terms of the application to the MEMS devicemanufacturing process.

Japanese Patent Application Publication (JP-P2005-104810A) discloses amethod of bonding a functional ceramic polycrystalline body and a singlecrystalline semiconductor material such as Si at room temperature. Inthis method, a metal thin film having reaction activity is formed of thesemiconductor material on the surface of the ceramic polycrystallinebody and the bonding is achieved through a reaction product layergenerated through reaction between the metal thin film and thesemiconductor material. This is an effective method for bonding of aceramic substrate with great surface roughness. Since this methodpresupposes reactivity between a bonding target substrate and a metallayer, target materials is restricted, and in some cases, heatingtreatment is required at the time of bonding.

On the other hand, a report by Takagi (Report by Mechanical Engineeringlaboratory, Number 189 (2000)) pointed out a possibility ofroom-temperature bonding for a bonding difficult material such as SiO₂,in which bonding strength cannot be obtained only by surface activationand pressure welding, by forming a metal film on the bonding surface.Specific methods for this have been proposed so far.

Japanese Patent Application Publication (JP-P2004-337927A) disclosesformation of a metal thin film on a bonding surface as a conventionalmethod of bonding ionic crystal substrates which are hard to be bondedby the room-temperature bonding method. An inert gas ion beam or aninert gas neutral atomic beam, and a metal ion beam or a metal neutralatom beam are radiated to the bonding surface in a vacuum, to form ametal thin film with a film thickness of 1 nm to 100 nm on a bondingsurface of each substrate.

Japanese Patent Application Publication (JP-P2004-343359A) discloses amanufacturing method of a surface acoustic wave device using aroom-temperature bonding method. In the method, the bonding is carriedout through interface bonding layers. A piezoelectric single crystallinesubstrate such as LiTaO₂ and an Al₂O₃ substrate or a crystallinesubstrate such as or Si are bonded by a surface activation process and apressure welding process without heat treatment at a high temperature.As one example in this method, the bonding is carried out by forming theinterface bonding layers of Si, insulating material and metal.

As mentioned above, it is difficult to bond SiO₂ material substrateswith a practical bonding strength by only a surface activation processand a pressure welding process, and it is effective to interpose a filmof bonding functional material such as metal at the bonding interface inorder to achieve practical bonding strength. However, sufficientconsideration has not been performed to proper conditions of acomposition of an interface bonding metal layer and concentrations ofelements of the layer. In particular, quantitative estimation has notbeen enough about proper amounts of metal atoms for achieving thepractical bonding strength. The formation of the interface bonding metallayer of an excessive thickness causes a disadvantage of an increase intime required for a process of forming the interface bonding layerlayer, so that a process cost increases and production efficiency dropsdue to an increase in a tact time. Also, the formation of the interfacebonding metal layer affects a physical property of a device bondinginterface to possibly lose performance of a device. For example, whenthe interface bonding metal layer is formed by using a physicalsputtering method such as an ion beam method, sputtering of the bondingsurface advances simultaneously with the formation of the interfacebonding metal layer as a radiation time becomes longer. As a result,surface roughness of the bonding surface increases so that the bondingstrength drops.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a device and amanufacturing method of the device, in which substrates are bonded morestrongly.

Another object of the present invention is to provide a devicemanufacturing method, in which a bonding condition of substrates ofbonding difficult materials is optimized.

Still another object of the present invention is to provide a device anda manufacturing method of the device, in which substrates of bondingdifficult materials (e.g. SiO₂ material substrates) are bonded atroom-temperature to have a practical bonding strength.

Yet still another object of the present invention is to provide aroom-temperature bonding apparatus, in which a proper bonding conditionof substrates of bonding difficult materials is optimized.

A device according to the present invention has a first substrate and asecond substrate. At this time, at least one metal element is present ina bonding interface between the first substrate and the secondsubstrate. The first substrate and the second substrate are bonded byuse of the metal element by a room-temperature bonding method.

It is preferable that an interface element existence ratio of one ormore metal elements should be 0.07 or above. The interface elementexistence ratio is more preferably 0.1 or above, and still morepreferably 0.2 or above. Here, the interface element existence ratiomeans a ratio of the number of atoms of one or more metal elements, tothe entire number of atoms present in the bonding interface. Morespecifically, the interface element existence ratio is defined as aratio of the number of atoms of one or more metal elements for thebonding, to a total of the number of atoms of component elements of thebonded substrates and the number of atoms of one or more metal elementsin the bonding interface. A part of the banding interface on the side ofeach of the substrates is a range from a bonding plane to the depth of 5nm in the substrate. This value is quantitatively given by a compositionanalyzer used in general, such as X-ray photoelectron spectroscopy (XPS)and energy dispersive fluorescent X-ray analysis (EDX analysis) using atransmission electron microscope.

When a plurality of metal elements exist in the bonding interface, theplurality of metal elements are exemplified by a set of metal elementsselected from the group consisting of: a set of iron and chromium, a setof iron and aluminum, a set of iron, chromium, and aluminum, a set ofiron, chromium, and nickel, and a set of iron, chromium, nickel, andaluminum. The metal atoms are distributed continuously ordiscontinuously in the bonding interface between the first substrate andthe second substrate, in the form of layers, in the form of islands orin the form of layers intermittently. A metal element other than theabove set of metal elements may be present in the bonding interface. Forexample, any of tungsten W, gold Au, silver Ag, copper Cu, tantalum Ta,and zinc Zn may be contained.

In the device of the present invention, the main component of the firstsubstrate may be oxide, e.g. silicon dioxide. Also, the first substratein the device of the present invention may be selected from the group ofconsisting of: a single crystalline material substrate, apolycrystalline material substrate, a glass substrate, a ceramicssubstrate, or a combination thereof. Alternatively, the main componentof the first substrate in the device of the present invention may befluoride, carbide, or nitride. Additionally, the main component of thefirst substrate may be the same as that of the second substrate.

The device manufacturing method according to the present inventionincludes sputtering the surface of a first substrate, attaching at leastone metal to the surface of the first substrate, and bonding the secondsubstrate to the surface of the first substrate at room temperature. Itis preferable that an interface element existence ratio of one or moremetal elements should be 0.07 or above. In the device manufacturingmethod of the present invention, the surface of the second substrate issputtered simultaneously with the sputtering of the surface of the firstsubstrate. Additionally, the device manufacturing method of the presentinvention includes attaching at least one metal to the surface of thesubstrate simultaneously with the sputtering of the surface of thesubstrate.

When a plurality of metal elements are contained in the bondinginterface, the plurality of metal elements are exemplified by a set ofmetal elements selected from the group consisting of: a set of iron andchromium, a set of iron and aluminum, a set of iron, chromium, andaluminum, a set of iron, chromium, and nickel, and a set of iron,chromium, nickel, and aluminum. In the bonding interface, a metalelement other than the above set of metal elements may be present. Forexample, any of tungsten W, gold Au, silver Ag, copper Cu, tantalum Ta,and zinc Zn may be contained.

The surfaces of the first substrate and the second substrate aresputtered with accelerated particles being radiated. On the other hand,at least one metal is emitted from a metal emitter, to which theaccelerated particles are radiated, and is attached onto the surfaces ofthe substrates. The metal emitter is exemplified by structural membersor component parts of internal units of the bonding apparatus, or asubstrate holding mechanism, a stage moving mechanism, and a substratepressure welding mechanism. In the device manufacturing method of thepresent invention, an amount of atoms of each of metal elements emittedtoward the surface of the substrate is properly adjusted to form aninterface bonding metal layer having practical bonding strength, bycontrolling parameters concerning the particles which are acceleratedand radiated toward the metal emitter. More in detail, it is preferablethat the velocity of particles accelerated and radiated should be setsuch that the interface element existence ratio of one or more metalelements is 0.07 or above. For example, when an ion beam is used forsputtering, the velocity of radiated particles can be controlled basedon an acceleration voltage applied to an ion beam source. Additionally,it is preferable that a radiation time during which the particles areradiated should be set such that the interface element existence ratioof one or more metal elements is 0.07 or above. In addition, it ispreferable that a radiation amount of particles radiated per unit timeshould be set such that the interface element existence ratio of theplurality of metal elements is 0.07 or above.

It is preferable that the bonding apparatus includes a vacuum chamberwhich produces vacuum atmosphere therein, a holding mechanism whichholds substrates under the vacuum atmosphere, a positioning mechanismwhich transfers the substrates to given positions, a physical sputteringmechanism which activates bonding surfaces of the substrates, and apressure welding mechanism which performs pressure welding by pressingthe activated bonding surfaces to each other. In addition, it ispreferable that any of an internal wall and structural members of thevacuum chamber, and structural members and component parts of theholding mechanism, the positioning mechanism, and the pressure weldingmechanism should be formed of material which emits metal particles of aplurality of metal elements in the composition when being sputtered bythe physical sputtering mechanism, and the metal particles attach to thesurfaces of the substrates such that an interface element existenceratio of the plurality of metal elements is 0.07 or above.

The metal elements are exemplified by a set of metal elements selectedfrom the group consisting of: a set of iron and chromium, a set of ironand aluminum, a set of iron, chromium, and aluminum, a set of iron,chromium, and nickel, and a set of iron, chromium, nickel, and aluminum.At the interface, a metal element other than the above set of metalelements may be present. For example, any of tungsten W, gold Au, silverAg, copper Cu, tantalum Ta, and zinc Zn may be contained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a room-temperature bonding apparatusaccording to an embodiment of the present invention;

FIG. 2 is a sectional view showing an initial state of substrates;

FIG. 3 is a sectional view showing a state of substrates when surfacesof the substrates are cleaned;

FIG. 4 is a sectional view showing a state of the substrates wheninterface bonding layers are formed;

FIG. 5 is a sectional view showing of the substrates when the substratesare bonded;

FIG. 6 is a sectional view showing the state of the substrates whenestimating bonding strength;

FIG. 7 is a graph showing change in coupling energy with respect toradiation time; and

FIG. 8 is a graph showing change in coupling energy with respect to aninterface element existence ratio in a bonding interface.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a room-temperature bonding apparatus according to thepresent invention will be described in detail with reference to theattached drawings. FIG. 1 shows a room-temperature bonding apparatus 1according to an embodiment of the present invention. As shown in FIG. 1,the room-temperature bonding apparatus 1 has a vacuum chamber 2, an iongun 3, an upper stage 5, and a lower stage 6.

The vacuum chamber 2 is a chamber which secures an internal space andisolates the internal space from the surroundings. A part or the wholeof the vacuum chamber 2 is formed of material which emits iron Fe,aluminum Al, and chromium Cr in a composition during sputtering. Thematerial is exemplified by stainless steel which contains iron Fe andchromium Cr in the composition. The vacuum chamber 2 has a lid (notshown) and is connected to a vacuum pump (not shown). The vacuum pumpdischarges gas from the vacuum chamber 2. The vacuum pump is exemplifiedby a turbo-molecular pump which performs discharge as a result offlicking gas molecules by a plurality of internal metal blades. The lidis used to close and open a gate connected to the vacuum chamber 2.

An upper stage 5 is formed in the shape of cylindrical column and issupported to be movable in parallel in a vertical direction. The upperstage 5 is formed of material which emits iron Fe, aluminum Al, andchromium Cr in a composition during the sputtering, and for example, isformed of stainless steel containing iron Fe and chromium Cr in thecomposition, and aluminum Al. The upper stage 5 has a dielectric layeron the lower end of the column, and when a voltage is applied betweenthe dielectric layer and a substrate 11, and absorbs and supports thesubstrate 11 toward the dielectric layer with electrostatic force. Theupper stage 5 is connected with a pressure welding mechanism, which isnot shown. The pressure welding mechanism moves the upper stage 5 in avertical direction in the vacuum chamber 2 in accordance with a useroperation.

A lower stage 6 is formed of material which emits iron Fe, aluminum Al,and chromium Cr in the composition during the sputtering, and is formed,for example, of stainless steel containing iron Fe and chromium Cr inthe composition, and aluminum Al. The lower stage 6 is connected with astage moving mechanism, which is not shown. The stage moving mechanismmoves the lower stage 6 in a horizontal direction and rotationally movesthe lower stage 6 around a rotation axis which is parallel to thevertical axis, in accordance with a user operation. The lower stage 6includes a substrate holder as a substrate holding mechanism formed ofaluminum, for example. Additionally, the substrate holding mechanism maybe provided with a dielectric layer on the upper end of the lower stage6, and absorb and hold the substrate 12 toward the dielectric layer withelectrostatic force which is generated by applying a voltage between thedielectric layer and the substrate 12.

The ion gun 3 is directed to the substrate 11 supported by the upperstage 5 and the substrate 12 supported by the lower stage 6. The ion gun3 accelerates and emits charged particles. The charged particles areexemplified by argon ions. The vacuum chamber 2 may further have anelectron gun, which is not shown. The electron gun is positioned towarda target, to which the charged particles are emitted from the ion gun 3,and emits electrons toward the target. Such electrons are used toelectrically neutralize the target which is positively charged by thecharged particles emitted by the ion gun 3.

A plurality of metal atoms are emitted from a metal emitter by receivingthe radiation of the charged particles. The metal emitter is provided inthe vacuum chamber 2, and may be structural members of the vacuumchamber, or surface members of the substrate holding mechanism includingthe upper and lower stages, the substrate moving mechanism, and thepressure welding mechanism.

FIGS. 2 to 5 show the states of the substrate 11 and the substrate 12when performing a room-temperature bonding process by theroom-temperature bonding apparatus 1. An operator firstly opens the lidof the vacuum chamber 2, to make the upper stage 5 hold the substrate 11and make the lower stage 6 hold the substrate 12. The operator closesthe lid of the vacuum chamber 2 and vacuums inside of the vacuum chamber2 to a vacuum atmosphere. Then, the operator operates the stage movingmechanism of the lower stage 6 and moves the lower stage 6 in thehorizontal direction such that the substrate 11 faces the substrate 12.

At this time, an inactive surface layer 21 has been formed on thesurface of the substrate 11 as shown in FIG. 2. The inactive surfacelayer 21 is composed of impurity which is attached on the substratesurface, products metamorphosed from the substrate, or a material topsurface layer in which bonds are terminated with oxygen and the like toset the surface to a poor reaction activity state. The inactive surfacelayer 22 is formed on the surface of the substrate 12 similarly in caseof the substrate 11. The inactive surface layer 22 is formed fromimpurity attached to the surface, products metamorphosed from thesubstrate material, or a material top surface layer in which bonds areterminated with oxygen to set the surface to a poor reaction activitystate.

The operator operates the ion gun 3 to emit the charged particles towardthe substrates 11 and 12 which are separated enough, as shown in FIG. 3.At this time, the substrates 11 and 12 are sputtered with the chargedparticles so that the inactive surface layers 21 and 22 are removed. Thecharged particles are also radiated to the metal emitter in the chamber(e.g. the vacuum chamber 2, the upper stage 5, and the lower stage 6).The metal emitter is sputtered through radiation of the chargedparticles, and emits a plurality of component metal atoms, e.g. iron Fe,aluminum Al, and chromium Cr, into the vacuum atmosphere. At this time,the operator adjusts radiation conditions of the charged particlesthrough change of the setting of operation parameters of the ion gun 3.Thus, emitted metal atoms form an intermediate material layer on thesurface of each substrate such that the interface element existenceratio takes a proper value in a range.

As shown in FIG. 4, when the radiation of the charged particles ends, anactive surface 24 of the substrate 11 is exposed and an active surface25 of the substrate 12 is exposed. The intermediate material layers 26are formed in the active surfaces 24 and 25. The intermediate materiallayers 26 are formed of elements emitted from the vacuum chamber 2, theupper stage 5, and the lower stage 6, e.g. iron Fe, aluminum Al, andchromium Cr.

As shown in FIG. 5, the operator operates the pressure welding mechanismsuch that the upper stage 5 lowers in the vertical direction and thesubstrate 11 and the substrate 12 come into contact with each other. Thesubstrate 11 and the substrate 12 are bonded at room temperature throughthe contact, and are strongly bonded into a unitary body. At this time,an inter-substrate material layer 28 is formed from the intermediatematerial layers 26 between the substrate 11 and the substrate 12. Theinter-substrate material layer 28 serves to increase bonding strength ofthe substrate 11 and the substrate 12.

It should be noted that in the above-mentioned example, the vacuumchamber 2 is formed of stainless steel, the upper stage 5 and the lowerstage 6 are formed of stainless steel and aluminum, and the substrateholding mechanism is formed of aluminum. However, a part of thesemechanisms may be formed of other materials. For example, the vacuumchamber 2 may be formed of aluminum. Also, the upper stage 5 and thelower stage 6 may be formed of aluminum alloy, for example. In addition,the substrate holding mechanism may be formed of stainless steel.

In a room-temperature bonding method according to the embodiment of thepresent invention, the device is produced by using the room-temperaturebonding apparatus 1. In the room-temperature bonding method according tothe present invention, the operation parameters of the physicalsputtering mechanism are determined, such that concentrations ofelements the inter-mediate material layer formed in the bondinginterface as a bonding functional material layer are within a properrange to attain practical bonding strength.

The operation parameters are exemplified by a voltage with which the iongun 3 accelerates the charged particles, a time during which the ion gun3 radiates the charged particles, and an amount of charged particleswhich the ion gun 3 emits (beam intensity, amount of current). Theoperator determines the operation parameters based on a relation of theoperation parameters and a measured value of the interface elementexistence ratio of a plurality of metal atoms present in the bondinginterface, such that the interface element existence ratio of the metalelements falls within a proper range. The proper range is exemplified by0.07 or above. Then, the substrates 11 and 12 are bonded after theinactive surface layers are removed by sputtering.

The interface element existence ratio of the plurality of metal elementsis defined as a ratio of the number of atoms of one or more metalelements involved in the bonding interface, to a total of the number ofatoms of component elements of the substrates and the number of atoms ofthe one or more metal elements in the bonding interface. That is to say,the ratio of the numbers of atoms of one or more metal elements in thebonding interface (e.g. iron Fe, chromium Cr, and aluminum Al), to thetotal of the number of atoms of component elements of the substrates inthe bonding interface (e.g. silicon Si and oxygen O in case of a quartzglass substrate) and the number of atoms of the one or more metalelements involved in the bonding interface (e.g. iron Fe, chromium Cr,and aluminum Al) is defined as the interface element existence ratio ofone or more metal elements involved in the bonding interface. A bondinginterface portion on side of each substrate is a range from a bondingplane to the depth of 5 nm.

The interface element existence ratio can be calculated based on elementconcentrations in the bonding interface measured by a general analysismethod. The measuring method of the element concentration is exemplifiedby X-ray photoelectron spectroscopy (XPS). In case of the XPS, signalintensity in proportion to the number of existing atoms is measured anda composition of component elements of the substrate and metal elementsis calculated from the signal intensities. Based on this analysisresult, the interface element existence ratio of the metal elements canbe calculated. The operator determines the operation parameters suchthat the interface element existence ratio of one or more metal elementsis 0.07 or above. Additionally, it is preferable that the operatorshould determine the operation parameters such that the interfaceelement existence ratio is 0.1 or above. It is more preferable that theoperator should determine the operation parameters such that theinterface element existence ratio is 0.2 or above. In addition, themetal atoms may also be iron and chromium. The metal atoms may furtherbe iron and aluminum. The metal atoms may further be iron, chromium, andnickel. The metal atoms may further be iron, chromium, nickel, andaluminum.

According to the room-temperature bonding method, a plurality of metalelements are appropriately present in the bonding interface of thedevice product, and the bonding interface has strong and practicalbonding strength. For this reason, the room-temperature bonding methodof the present invention makes it possible to perform theroom-temperature bonding process of substrates formed of materials whichare hard to be bonded at room temperature so as to have stronger andpractical bonding strength without causing deterioration in deviceperformance and any drop in productivity due to excessive formation ofthe intermediate material layer and redundancy of a process time. Thesubstrate material is exemplified by oxide, nitride, carbide, fluoride,and metal. The oxide substrate is exemplified by an SiO₂ substrate andan Al₂O₃ substrate. The SiO₂ substrate is exemplified by substrates ofquartz, synthetic quartz, Pyrex (registered trademark), glass, andquartz glass. The Al₂O₃ substrate is exemplified by substrates ofsapphire and alumina. The nitride substrate is exemplified by a siliconnitride SiN substrate and a titanium nitride TiN substrate. The carbidesubstrate is exemplified by a silicon carbide SIC substrate and atitanium carbide TiC substrate. The fluoride substrate is exemplified bya calcium fluoride CaF₂ substrate and a magnesium fluoride MgF₂substrate. The metal substrate is exemplified by a substrate of simplemetal and alloy. The substrate is further exemplified by substrates ofan optical crystal, piezoelectric material and magneto-strictivematerial. The optical crystal substrate is exemplified by substrates ofCaCO₃, (YVO₄, and YAG. The piezoelectric material and magneto-strictivematerial are exemplified by PZT. The room-temperature bonding method ofthe present invention is applicable even when two substrates to bebonded at room temperature are formed of different materials among theabove-mentioned materials, and makes it possible to bond the twosubstrates at room temperature more strongly without causingdeterioration in device performance and a drop in productivity due toexcessive formation of the intermediate material layer and redundancy ofprocess time.

In case of the device products manufactured by the room-temperaturebonding method, an interface element existence ratio of one or moremetal elements present in the bonding interface can be measured. Themeasuring method is exemplified by EDX analysis by using a transmissionelectron microscope. The X-ray photoelectron spectroscopy (XPS)mentioned above and the EDX analysis by using the transmission electronmicroscope can quantitatively analyze element concentrations though theapproaches are different, and there is a correlation between the resultsof concentration analysis derived from the respective analysis methods.Based on this correlation, it is possible to analyze and estimate aninterface element existence ratio of one or more metal elements presentin the bonding interface and confirm that the interface elementexistence ratio is within a proper range as bonding conditions.

Bonding strength of the substrates bonded at room temperature can beestimated by using biding energy of a bonding section. The couplingenergy can be estimated with a well-known blade insertion method. Theblade insertion method is disclosed by Maszara et al. (J. Appl. Phys. 64(10) pp. 4943-4950 (1988)), for example. FIG. 6 shows the state ofsubstrates when coupling energy is to be measured by the blade insertionmethod. That is, the operator inserts a razor blade 43 in the bondinginterface between a substrate 41 and a substrate 42 bonded at roomtemperature. At this time, the substrate 41 and the substrate 42 areseparated from each other to generate a crack 44. The operator measuresan extension length of the crack 44. Coupling energy γ per unit area ofone surface in the bonding interface is expressed by the followingequation by using the extension length L of the crack 44, a half value yof the thickness of the razor blade 43, and a thickness t of each of thesubstrates 41 and 42, and a Young's modulus E of the substrates 41 and42:

$\gamma = \frac{3\; {Et}^{3}y^{2}}{8\mspace{14mu} L^{4}}$

The coupling energy γ shows that bonding strength is greater and thesubstrates are more difficult to be separated, as the value is larger.

As a reference of practical bonding strength, the value of the couplingenergy γ is 0.1 J/m² or above, for example. Also, the coupling energy γbeing 0.1 J/m² or above shows that the substrate 41 and the substrate 42are bonded not to be separated when dicing of the substrate 41 and thesubstrate 42 is gently performed, though the bonding strength is weak.The coupling energy γ being 0.5 J/m² or above shows that the substrate41 and the substrate 42 are bonded with practical bonding strength notto be separated when dicing of the substrate 41 and the substrate 42 isperformed at high speed.

While the coupling energy increases with increase in metal elementswhich generate a bonding function, excessive process causes decrease inthe coupling energy. A substrate is known to increase surface roughnessof a surface as being sputtered. Furthermore, a substrate is known todecrease bonding strength as surface roughness of a surface to be bondedat room temperature increases. FIG. 7 shows a relation of couplingenergy and radiation time (time for sputtering) during which the chargedparticles are radiated to the substrate, in bonding of substrates by theroom-temperature bonding apparatus. The graph shown in FIG. 7 shows thatmetal elements involved in the bonding interface as bonding functionalmaterial increases with radiation time so that the coupling energyincreases when radiation time is shorter than a certain value, and thatthe effect of increase in surface roughness due to the extension ofradiation time becomes greater so that the coupling energy decreases,when the radiation time is longer than that value. That is to say,radiation time has a proper range (an upper limit and a lower limit).

FIG. 8 shows a relation of bonding strength and an interface elementexistence ratio in a bonding interface. The interface element existenceratio x is calculated based on a result of measurement of elementconcentrations in the bonding interface measured from a surface of thesubstrate before being bonded by using the XPS. The coupling energy y isgenerally expressed by the following equation by using the interfaceelement existence ratio x:

y=3.336x−0.242

That is to say, the graph shown in FIG. 8 shows a correlation betweenthe interface element existence ratio x and the coupling energy y. Thegraph of FIG. 8 also shows that the coupling energy becomes 0.0 J/m² orabove so that substrates can be bonded at room temperature, when theinterface element existence ratio is 0.07 or above. The graph of FIG. 8further shows that the coupling energy reaches 0.1 J/m² or above, andthe substrates are bonded not to be separated if dicing is gentle, whenthe interface element existence ratio is 0.1 or above. The graph of FIG.8 further shows that the coupling energy becomes 0.5 J/m² or above andthe substrates are bonded not to be separated even if dicing isperformed at high speed, when the interface element existence ratio is0.2 or above. That is to say, it is possible to manufacture a devicehaving practical bonding strength of 0.5 J/m² or above without causingperformance degradation, by performing sputtering and bonding underoperation conditions which do not excessively exceed the operationparameters that the interface element existence ratio of 0.2 or abovecan be achieved. In addition, a correlation is calculated withoutdepending on a specific measuring method of measuring elementconcentrations in the bonding interface. An interface element existenceratio calculated by other measuring methods also shows a correlationwith the coupling energy y. Additionally, the interface elementexistence ratio can be calculated based on a result of measurement ofelement concentrations in the bonding interface of the substrates. Themeasuring method is exemplified by the EDX analysis using a transmissionelectron microscope. The interface element existence ratio calculatedfor the bonded substrates also shows a correlation with the couplingenergy y in the same way.

A device manufactured according to the embodiment of the presentinvention is manufactured by the room-temperature bonding apparatus 1.The device is exemplified by a micro-machine, a pressure sensor, and amicro motor. The device has two substrates to be bonded at roomtemperature. The substrates are formed of materials which are hard to bebonded at room temperature. The material is exemplified by oxide,nitride, carbide, fluoride, and metal. The oxide is exemplified by SiO₂and Al₂O₃. The SiO₂ is exemplified by quartz, synthetic quartz, Pyrex(registered trademark), glass, and quartz glass. The Al₂O₃ isexemplified by sapphire and alumina. The nitride is exemplified bysilicon nitride SiN and titanium nitride TiN. The carbide is exemplifiedby silicon carbide SiC and titanium carbide TiC. The fluoride isexemplified by calcium fluoride CaF₂ and magnesium fluoride MgF₂. Themetal is exemplified by simple metal and alloy. The material is furtherexemplified by an optical crystal, and piezoelectric material andmagneto-strictive material. The optical crystal is exemplified by CaCO₃,YVO₄, and YAG. The piezoelectric material and magneto-strictive materialare exemplified by PZT. The two substrates are formed of the samematerial or formed of different materials.

An inter-substrate material layer is formed in the bonding interfacebetween the two substrates. The inter-substrate material layer may beplaced in a part of the bonding interface or placed in the entire partof the bonding interface. The inter-substrate material layer may beformed of a plurality of metal elements. The metal elements areexemplified by iron, aluminum, and chromium. The inter-substratematerial layer is formed such that the metal elements in the bondinginterface have the interface element existence ratio of 0.07 or above.It can be measured by using an energy dispersive X-ray fluorescenceanalyzer (EDX) that the device has the inter-substrate material layer.The device according to the present invention has stronger bondingstrength in the interface bonded at room temperature with aninter-substrate material layer. Additionally, it is more preferable thatthe inter-substrate material layer should be formed such that the metalatoms in the bonding interface have the interface element existenceratio of 0.1 or above, and it is still more preferable that theinter-substrate material layer should be formed such that the interfaceelement existence ratio is 0.2 or above.

1. A device comprising: one or more metal elements; a first substrate; asecond substrate bonded to said first substrate at room temperaturethrough said one or more metal elements, wherein an interface elementexistence ratio of said metal elements is 0.07 or above.
 2. The deviceaccording to claim 1, wherein the interface element existence ratio ofsaid one or more metal elements is 0.1 or above.
 3. The device accordingto claim 2, wherein the interface element existence ratio of said one ormore metal elements is 0.2 or above.
 4. The device according claim 1,wherein said device comprises a plurality of said metal elements, saidplurality of metal elements is a set of metal elements selected thegroup consisting of: a set of iron and chromium, a set of iron andaluminum, a set of iron, chromium, and aluminum, a set of iron,chromium, and nickel, and a set of iron, chromium, nickel, and aluminum.5. The device according to claim 1, wherein said first substrate hasoxide as a main component.
 6. The device according to claim 5, whereinsaid first substrate has silicon dioxide as the main component,
 7. Thedevice according to claim 6, wherein said first substrate is formed of amaterial selected the group consisting of single crystal material,polycrystalline material, glass, and ceramics.
 8. The device accordingto claim 1, wherein said first substrate has fluoride as a maincomponent.
 9. The device according to claim 1, wherein said firstsubstrate has carbine as a main component.
 10. The device according toclaim 1, wherein said first substrate has nitride as a main component.11. The device according to claim 1, wherein a main component ofmaterial of said first substrate is the same as that of material of saidsecond substrate.
 12. A method of manufacturing a device, comprising:performing sputtering on a surface of a first substrate; attaching oneor more metal elements onto the surface of said first substrate; andbonding a second substrate to the surface of said first substrate atroom temperature, wherein an interface element existence ratio of saidone or more metal elements is 0.07 or above.
 13. The method according toclaim 12, wherein the interface element existence ratio of said one ormore metal elements is 0.1 or above.
 14. The method according to claim13, wherein the interface element existence ratio of said one or moremetal elements is 0.2 or above.
 15. The method according to claim 12,further comprising: performing the sputtering on a surface of saidsecond substrate at the same time that the surface of said firstsubstrate is sputtered.
 16. The method according to claim 12, whereinsaid performing is performed as the same time as said attaching.
 17. Themethod according to claim 12, wherein said performing comprises:radiating accelerated particles to sputter the surface of said firstsubstrate, and said attaching comprises: attaching said one or moremetal elements on the surface of said first substrate, wherein said oneor more metal elements are emitted from a metal emitter to which theaccelerated particles are radiated.
 18. The method according to claim17, wherein said metal emitter is at least one of a bonding apparatus,structural members and parts of a substrate holding mechanism a stagemoving mechanism, and a substrate pressure welding mechanism disposed insaid bonding apparatus.
 19. The method according to claim 17, wherein aparameter of the radiation of the particles is a velocity of theparticles.
 20. The method according to claim 17, wherein a parameter ofthe radiation of the particles is a radiation time during which the 10particles are radiated,
 21. The method according to claim 17, wherein aparameter of the radiation of the particles is an amount of theparticles radiated onto 15 the surface of said first substrate per aunit time.
 22. The method according to claim 12, wherein a plurality ofsaid metal elements are attached, and said plurality of metal elementsare a set of metal elements selected the group consisting of: a set ofiron and chromium, a set of iron and aluminum, a set of iron, chromium,and aluminum, a set of iron, chromium, and nickel, and a set of iron,chromium, nickel, and aluminum.
 23. A room-temperature bonding apparatuscomprising a vacuum chamber, in which a vacuum atmosphere is generated;a substrate holding mechanism configured to hold substrates to be bondedunder the vacuum atmosphere; a stage moving mechanism configured to moveone of said substrates to a position; a physical sputtering mechanismconfigured to activate bonding surfaces of said substrates; and apressure welding mechanism configured to perform a pressure weldingprocess by pressing the activated bonding surfaces to each other,wherein a plurality of metal elements are emitted in a predeterminedcomposition from at least one of structural members and parts of saidbonding apparatus, said substrate holding mechanism, said stage movingmechanism, and said pressure welding mechanism disposed in said bondingapparatus through sputtering by said physical sputtering mechanism. 24.The room-temperature bonding apparatus according to claim 23, whereinsaid plurality of metal elements are a set of metal elements selectedthe group consisting of: a set of iron and chromium, a set of iron andaluminum, a set of iron, chromium, and aluminum, a set of iron,chromium, and nickel, and a set of iron, chromium, nickel, and aluminum